The term drug discrimination refers to the ability of animals to learn to respond differentially as a function of drug state. Such discriminations are very rapidly learned with certain psychoactive drugs; these are the same drugs which can produce state-dependent learning. Neither phenomenon is produced by drugs which lack psychoactive properties. The general objective of the proposed research is to increase our knowledge of the drug discrimination phenomenon. Experiments currently planned include: (1) Measuring the discriminative properties of a broad range of drugs, many of which have not been previously studied. (2) Employing brain lesions to explore central mechanisms which may be involved in drug discrimination phenomena. (3) Testing the hypothesis that a correlation exists between the discriminability of drugs and their potential for abuse. (4) Determining the conditions under which state-dependent learning is a prominent effect in operant tasks. (5) Using drug discrimination techniques to test the central effects of narcotics and narcotic antagonists. Most of the proposed research will involve drug discrimination studies in a well-standardized T-maze task.